Acid catalysts containing, in addition to a carrier, a noble metal of the platinum family and at least one additional metal selected from the group consisting of tin, germanium and lead, have been known for a long time. Thus they are disclosed in the French Pat. No. 2,031,984.
The carriers are generally selected from oxides of metals from groups II, III and/or IV of the periodic classification of elements, such for example as the oxides of magnesium, aluminum, titanium, zirconium, thorium or silicium, either alone or admixed with one another or with oxides of other elements of the periodic classification, such as boron. Coal also can be used. Other carriers may consist of zeolites or molecular sieves of the X or Y type, of the mordenite or faujasite type or of the ZMS-5, ZMS-4, ZMS-8 type etc. . . . or of mixtures with zeolite material of oxides of metals from groups II, III and/or IV.
The preferred carrier for the reactions of reforming, of aromatic hydrocarbons production or of isomerizing paraffinic or aromatic hydrocarbons, is alumina, advantageously of a specific surface from 50 to 600m.sup.2 per gram, preferably from 150 to 400m.sup.2 /g.
The catalyst is usually prepared by a conventional method consisting of impregnating the carrier by means of solutions of compounds of the metals to be introduced. The impregnation may be performed with a common solution of these metals or with a separate solution for each metal. When using several solutions, intermediate drying or calcination steps may take place. Usually, a final calcination is conducted, for example at a temperature from about 500.degree. to 1000.degree. C., preferably in the presence of free oxygen, for example by air scavenging.
Platinum (and optionally another noble metal from the platinum group) may be incorporated in the carrier by impregnating said carrier with an adequate aqueous or non aqueous solution containing a salt or a compound of the noble metal. Platinum is generally introduced into the carrier as chlorophatinic acid.
The element selected from the group consisting of tin, germanium and lead, may be introduced as metal compound such as a tin chloride, bromide or nitrate, a lead halide, nitrate, acetate or carbonate, germanium chloride or oxalate in aqueous solution etc....
The halogen of the catalyst may originate from one of the metal halides when the metal is introduced as halide or it may be introduced as hydrochloric acid or hydrofluoric acid, ammonium chloride, ammonium fluoride, chlorine gas or a hydrocarbon halide, e.g. CCl.sub.4, CH.sub.2 Cl.sub.2 or CH.sub.3 Cl etc. . . .